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Thursday, September 20, 2012

Dengue vaccine- "Hit or miss"

Hello everyone,

Before i begin this blog session (Not exactly), i have a small statement of thanks giving to make. The blog started at the beginning of this year with as few as 5-7 page hits a week. That was pretty impressive for a beginner in a sub-domain blog. From then i have been writing, and evolved writing about things that i wanted to express about or what i thought was worth a share. And now i have nearly 700 page hits a week after 9 month of inception. Needless to say, I'm very impressed and happy about it, there is one thing that i haven't still achieved. Am referring to the discussion here. The main purpose of the internet share was to evoke thoughts from various people, which hasn't come true yet. But then i have positives to take from this blog.

The 2nd thing that i have to say is that i have decided to move my blogging to my new webpage soon. I don't wish to have an hibernating domain name. And i will see you people soon there.

As always first a small news on science as a whole. Unlike always where a top science news was something from physics and space, this time i have something special on nanotechnology. The researchers from University of California have been able to create nano-models from hydrogels. Ooh, what is the big deal in that. The creation is not a big deal. But the time they take to create this is. Shaochen Chen labels the biofabrication technology, called dynamic optical projection stereolithography (DOPsL). For more information and source go here and here.

For a long time now, the biology science has been studying molecules by the process of tagging. The problem is however, the tags are usually irreversible (Not in the strictest sense) and removal may cause damage to the protein which excludes it from recycling. A new innovative method from Michael Burkart's laboratory, gets around this problem. They find a method to attach chemical labels into proteins and can subsequently remove them at will. An incredible leap is flexibility of what can be attached. This can be dyes, purification agents or metabolic products. The study can be found here. Burkart said "We've shown that we can do this, at least four or five times, without any degradation of the protein" (Source).

You all should be aware of the great scientific debate on the link between XMRV and Chronic fatigue syndrome. The whole world of science was split into debate (Read more here). With every new study the strength of association seemed to have weakened and the weight of evidence was in favor of "No". The final blow to this has come from the NIH project costing nearly US$2.3-million. This study by W. Ian Lipkin is the feature of week in the world of science (The story is well covered in nature).

I have two equally interesting stories in microbiology that i want to blog about here in a bit of detail. I don't know which paper need to be summarized and what need to be elaborated. The first paper of interest to me is about the recent findings on "CYD tetravalent dengue vaccine" and the 2nd is on "pyridomycin" as potential anti-TB agent. Finally i decide that the vaccine story will go on a bit detail and i just tell you about the new drug.

Photo 2: AFB in sputum

Tuberculosis is a global health problem and more common in the developing and under developed nations. Mycobacterium tuberculosis, accounting for almost 2 million deaths annually. The problem? Its diagnosis and treatment equally. Most of the bacteria are well identifiable in the laboratory by culture methods which takes less than 3-4 days. That means the pathogen is recovered, tested and appropriate treatment is initiated. But the stealth nature of TB, which replicates something like once in 20 Hrs is a slow grower. Even with the best culture medium support, the bacteria takes about 3-4 weeks to make a colony appearance. Ooh, that's nasty and to wait for such a long time is not on the good side of patient. One of the gold standards is still Acid fast staining method. However, the smears are positive only if the bacterial load is more than 10,000 bacilli/ml. Needless to say we are coming up with new technologies but its far from reach in routine resource poor laboratories. The POCT (point of care testing) is still AFB.

The second problem that the pathogen presents is treatment. The covertness of bacilli is the problem again. The very long replicative cycle which is difficult to attack is the main problem. And we have the special problem of dealing with a special C50 based mycolic acid based cell wall to deal with. The number of drugs, hence in the clinician's armor is far too less. Natural products such as the rifamycin group (For mechanism of action, here). The paper in EMBO molecular medicine by Ruben CH etal, has discussed the importance of Pyridomycin, as a potential anti-TB agent. The compound is naturally produced by Dactylosporangium fulvum. The study concluded that the compound is a competitive inhibitor of the NADH-binding site of InhA, NADH-dependent enoyl-[Acyl-Carrier-Protein] reductase, the target of the two anti-tuberculosis pro-drugs isoniazid and ethionamide. The MIC was studied using resazurin reduction microtitre assay. And what impressed me is the MIC was just 0.39 for TB. I strongly recommend you read the paper (Its open access).

Dengue, another product of mosquito bites is a viral disease caused by dengue virus. The virus is a flavivirus member belonging to arbovirus group. There are 4 important antigenic subtypes of dengue virus- DENV-1, DENV-2, DENV-3 and DENV-4. The types have a homologous sequence with more than 70% similarity. The major difference lies in their surface antigenic display. For a in depth discussion on structure of dengue virus go here. The virus encodes a 11 Kbp genome, which translates into a single unspliced polyprotein. The coding (from 5'-3') goes as follows

5'-C- prM-E-NS1-NS2A-NS2B-NS3-NS4A-NS4B-NS5-3'

A vaccine is highly desirable against a pathogen that affects almost 3/4th of the world especially the tropics. But there seems to be a problem. On primary infection the virus causes production of antibodies. Nothing new in that. A second infection subsequently by any serotype usually tricks the immune system and causes an improved response. Remember the similarity quotient is high and so cross reacting antibody is present. The antibodies bind to the surface proteins of virus, but doesn't inactivate the virus. The immune system responds with macrophages in massive concentration leading to "Antibody-Dependent Enhancement" (ADE) of a viral infection. A cytokine response is initated and endothelial tissue become permeable which results in Dengue Haemorrhagic Fever (DHF) and fluid loss from the blood vessels. So the catch of the problem is that when immune system is sensitized to the dengue surface antigens, the body responds inappropriately causing more grave condition. In this case the well known fact is that NS1 cross-reacts with human platelets and endothelial cells and cause their damage and dysfunction. Thats is why, the platelets are effected when there is a dengue inection. The Fig 2 below explains the auto-immune action.

Several attempts were made in making a vaccine. However the trick was to induce an antibody independent immunity, in other words cellular immunity. CYD TDV is composed of four recombinant, live, attenuated vaccines (CYD-1-4) based on a yellow fever vaccine 17D (YFV 17D) backbone, each expressing the pre-membrane and envelope genes of one of the four dengue virus serotypes. The vaccine is produced using vero cell lines. The phase 2b study was conducted on 4002 enrolled Thai children from Feb 5, 2009, and Feb 5, 2010. The vaccine proved to have a efficacy o more than 70% in case of DENV-1, 3 and 4. But, the mark was not hit with DENV-2 for unknown reasons. Now before i get to dealing with type 2 let me try to ask "Is 70% efficacy enough". Many scientists over the world opinion it as not good enough response.

"The question is, is the glass half-empty or half-full? True the vaccine shows protection against three of the strains, but unfortunately and this is very disappointing, overall it was not effective, showing no protection for the most common dengue strains circulating in Thailand. This is a very disappointing result for people interested in dengue that have been following this research -- we were all hoping for a much stronger result. The current data suggest that there is still more work needed on dengue vaccine development."

My point is there is no doubt that there is scope for improvement. But i have doubt if it is disappointing. I would be more optimistic that this is encouraging. Everyone remembers the malaria RTS,S/AS01 vaccine. It had less than 50% efficiency, but yet considered to be a achievement. The argument comes from a mathematical reasoning that the protected 50% will reduce the transmission to a high degree. Let me explain. The mosquito transmission from one human to another is quite low, and some sources claiming it to be less than 10%. So once you reduce the number of source patients to less than 50% by vaccination, theoretically the transmission in the community as a whole is less than 10%. After all our goal is to achieve herd immunity.

The 2nd part of the question mark is on DENV-2. Am sure the team is working on that. This is a multi national study and many of the data is expected to reach completion on 2014. So maybe the genetics of the study population doesn't respond well to DENV-2. My remark is that the conclusions disappointing and Not effective is inappropriate. Moreover this study shows that dengue vaccine is indeed a possibility by getting around an antibody independent mechanism. As the authors remark in their conclusion in Lancet, "In the context of WHO goals to reduce dengue mortality by at least 50% and the morbidity rate by at least 25% by 2020, this study represents a major milestone."

Just a point of note to people who think dengue is unbeatable right now, we have the Wolbachia as a tool under study .